Notecards can be a great way to communicate with SL friends who are currently offline. You can drop a notecard on their profile and have it delivered to them to be read later when they log in.
The only problem is that when notecards are delivered this way the User Interface quietly files them away in inventory with no notification. Most times the recipient isn't aware that you tried to communicate with them until days later when the might discover an unexplained Notecard in their inventory.
I think that when a notecard is delivered to an account that is currently offline the recipient ought to be notified of that fact when they log in. I propose a dialoge box that states the notecard's filename and the sender and has three buttons: [Accept]: Puts the notecard directly in inventory. [Delete]: Throws the notecard away. [Open Now]: Opens the notecard.
One of the things that makes email so useful is that the recipient can read and respond to it at their leisure without having to be present when the email is sent. This kind of asynchronous communication is missing from SL, other than IM's which is a very clunky way to do this kind of thing.
Let me know what you think.